A gas turbine, also called combustion turbine, is a rotary drive machine which transforms the potential energy contained in the combusted gas flow into mechanical energy transmitted to a rotating shaft.
Turbines normally comprise a compressor or turbo-compressor into which air removed from the outside is brought. In the combustion chamber(s) various nozzles therefore feed the fuel which is mixed with the air to form an air-fuel triggering mixture.
The compressor, of the axial type, is entrained by the turbine, which supplies mechanical energy transforming the enthalpy of the combusted gases in the combustion chamber.
In some applications, the expansion jump is subdivided into two partial jumps, each of which takes place inside a turbine. The high-pressure or first-phase turbine, downstream of the combustion chamber, entrains the compressor, whereas the low-pressure or second-phase turbine, which collects the gases coming from the high-pressure turbine, is then connected to the user.
The compressor, the combustion chamber, the various turbine phases, the outlet shaft, the control system and ignition system, form the essential parts of a gas turbine plant.
As far as the functioning of a gas turbine is concerned, it is known that the air is sucked into the compressor through an inlet duct. In this duct, the air has low-pressure and low-temperature characteristics whereas its pressure and temperature increase as it passes through the compressor. The air then penetrates the combustion chamber(s), where it takes part in the combustion process with a consequent further considerable increase in temperature.
The fuel can be of the liquid or gaseous type and is introduced into the combustion chamber by means of specific nozzles. The triggering of the combustion, when the machine is activated, is obtained by means of spark plugs.
When it leaves the combustion chamber, the high-pressure and high-temperature gas, resulting from the combustion, reaches the turbine phases where it yields part of the energy accumulated in the compressor and in the combustion chamber and then flows to the outside through outlet channels.
In gas turbines, the turbine wheels must normally be subjected to cooling to keep the functioning temperatures compatible with the characteristics of the material of the same turbine wheels. The cooling is normally effected by means of flows of air, extracted from the axial compressor and suitably conveyed, which is then sent radially towards the outside through the interspaces between the surface of each turbine wheel and the stator. These air flows touch the surfaces of the turbine wheels, cooling them.
In particular, in some applications of the known type, in correspondence with the compressor delivery, a labyrinth sealing system is installed, i.e. consisting of channels with a series of sectional enlargements and contractions, or another type of sealing system. The air flow which is drawn from said sealing, effects the cooling of the front surface (compressor side) of the first-phase turbine wheel.
Sometimes, as a result of an excessive sealing of the labyrinth system or for other reasons, the cooling air flow can be insufficient. As a consequence of this, there is an increase over the acceptable limits of the temperatures measured in correspondence with the front interspace between the first-phase turbine wheel and the stator, and therefore also an increase in the temperature of the turbine wheel itself. This can lead to a drastic reduction in the operative life of the turbine wheel.
In order to bring said temperatures back to acceptable values, it is therefore necessary to increase the cooling air flow coming from the compressor, this increase being effected, in turbines of the known type, by increasing the clearances of the sealing system or creating one or more holes having a suitable diameter in the inner casing in correspondence with the compressor delivery and cooling air channels. In order to be activated in pre-existing turbines, both solutions, however, require the dismantling of various components, which implies higher costs and also lengthy periods of inactivity for the whole machine.